From the standpoint of resource and energy savings, for example in the automotive industry, it is desirable to reduce vehicle body weight. However, attempts to reduce weight by reducing the thickness of materials result in deterioration in strength. For example, in the case of quarter panels for automobiles, such attempts have created problems such as low strength at various points (particularly flexural strength). Insufficiently rigid outer panels cause buckling and bending with minimum applied stress.
In order to solve such problems, it is necessary to develop a suitable way to reinforce such structures. Reinforcement with a heavy metal panel is contradictory to the purpose of vehicle weight reduction. Hence, it has been proposed to reinforce the outer panels entirely or partly with lightweight resin materials.
Reinforcement comprised of thermosetting adhesive layers and a reinforcing material has been applied to steel panels for use in the production of the body of automobiles. Such reinforcement provides increased strength against force applied to the outside surfaces of the automobile body. Such reinforcement further serves to decrease vibration and corrosion of the metal panels.
For example, U.S. Pat. No. 4,369,608 describes a door for an automobile wherein a main reinforcing member made of a thermosetting resin is bonded to the inner side of an outer metal panel of the door to improve the strength and rigidity of the door. A multilayer sheet-like auxiliary reinforcing member, preferably made of a glass fabric, is bonded to the main reinforcing member. A wave-like or bead-like projection is provided on the main reinforcing member, which projection functions as a rib of the metal panel to be reinforced and serves to increase the reinforcement effects of the reinforcing sheet.
Typically, the reinforcing sheet is adhered under pressure, for example to the back surface of the metal panel to be reinforced, and thereafter cured by usual heating methods, e.g. in a hot air circulation type heating furnace, an infrared ray heating furnace or a radio frequency induction heating furnace. This heat curing treatment can be carried out simultaneously at a stage in a vehicle assembly line at which painted metal panels, having a reinforcing sheet thereon, are baked to cure the paint.
The reinforcing sheet may be premolded to conform its shape with that of the metal panel to be reinforced. Preferably, the reinforcing sheet has sufficient flexibility so as to conform to the shape of the metal panel upon its application to the metal panel without premolding. In either case, in order for the reinforcing sheet to decrease vibration and corrosion of the metal panel and to provide enhanced strength and rigidity to the panel, it is necessary that the reinforcing sheet maintain good contact with the panel to be reinforced both before, during and after curing of the thermosetting adhesive layer.
U.S. Pat. No. 4,444,818 describes a reinforcing sheet containing a first thermosetting adhesive layer, a second thermosetting adhesive layer with a reinforcing material embedded therein, and a protective film covering the entire reinforcing sheet. This sheet is used to secure a flattened tubular material to the metal panel to be reinforced. Upon heating the sheet to cure the thermosetting adhesive layers, the flattened tubular material recovers its original tubular form to provide a wave-like or ribbed construction for the reinforcing sheet.
According to U.S. Pat. No. 4,444,818, the reinforcing material is embedded in the second thermosetting adhesive layer and provides increased reinforcing effect to the metal panel. Reinforcing materials include cloths of inorganic fibers, such as of glass or asbestos fabric; cloths of organic fibers, such as of flax, cotton, nylon, polyester or polypropylene fiber; plastic films such as of polyester or nylon film; paper such as kraft paper; nonwoven fabrics such as of polyester fiber or polypropylene fiber; and metal foils such as of aluminum, iron, copper or zinc foil.
U.S. Pat. No. 4,444,818 further describes the use of a protective film on the side of the reinforcing sheet opposite that of the metal panel to be reinforced. According to the patent, the protective film facilitates separation of the reinforcing sheet when stored in the form of a rolled tape, participates in the reinforcement of the metal panel to be reinforced, and improves the moisture resistant characteristics of the fabric reinforced resin layer. Various films, such as polyester, polyethylene, nylon, polyvinyl chloride, and polypropylene films, are described.
However, the use of conventional reinforcing adhesive sheets containing thermosetting resins for reinforcing a metal panel, including those adhesive sheets containing a protective film as described in U.S. Pat. No. 4,444,818, has drawbacks.
Thus, reinforcing adhesive sheets containing thermosetting resins have a tendency readily to absorb moisture, including moisture that is naturally present, for example in the atmosphere. The absorbed moisture adversely affects the thermosetting adhesive layer of the reinforcing sheet and, therefore, the adhesion of the reinforcing sheet to the metal panel upon curing of the thermosetting adhesive layer. Such adverse effects are particularly pronounced when the metal panel, with attached reinforcing sheet, is stored for a number of months, especially under humid conditions, before curing of the adhesive layer. It is believed that such adverse effects stem from decomposition of the curing agents in the thermosetting adhesive layer such that gas forms and is trapped in the thermosetting adhesive layer during curing. Due to the trapped gas, the adhesive layer is not as cohesive and the reinforcing sheet tends to fall from the metal panel.
Even the protective film described in U.S. Pat. No. 4,444,818 does not adequately prevent or solve the problems caused by the absorption of moisture by the reinforcing sheet before curing. One skilled in the art can readily appreciate that the thin polyester, polyethylene, nylon, polyvinyl chloride or polypropylene films described in U.S. Pat. No. 4,444,818 cannot maintain complete impermeability to atmospheric moisture over an extended period of time due to their relatively poor humidity barrier properties.
Moreover, the location of the protective film, presumably positioned to protect the entire reinforcing sheet including the expandable core, necessarily involves drawbacks. The location of the protective layer as the outside layer of the reinforcing sheet subjects the protective layer to wear and tear during handling and storage of the reinforcing sheet. In addition, the adverse consequences resulting from any damage to the protective layer, such as a worn spot, tear or hole, would be aggravated by its placement on top of the absorptive woven or nonwoven reinforcing material. Capillary action of the reinforcing material magnifies the amount of moisture absorbed by the thermosetting adhesive layer such that the area affected by any damage to the protective layer would be proportionately larger than the area of the damage itself. Furthermore, capillary action from the unprotected sides of the reinforcing material when the protective layer is placed on top of the absorptive reinforcing material is sufficiently strong adversely to affect the entire adhesive layer. As a result of the absorption of moisture by the adhesive layer, the reinforcing sheet would tend to fall from the metal panel during curing of the adhesive layer.
To replace the polyester protective layer of U.S. Pat. No. 4,444,818 with a material that has greater impermeability to moisture and greater resistance to damage, such as a metal film, would be expected to induce additional drawbacks. Such metal foil would necessarily be more inflexible than the polyester layer and would tend to buckle and separate from the underlying reinforcing material during handling and upon conforming the shape of the reinforcing sheet to the shape of the panel to be reinforced. Such buckling and separation of the metal protective layer from the underlying reinforcing material would create tunnels which would allow moisture to be even more readily absorbed by the reinforcing sheet.
It can thus be readily appreciated that provision of a reinforcing sheet which is resistant to moisture and the tendency to absorb atmospheric humidity and eliminates the previously discussed problems would be a highly desirable advance over the current state of panel reinforcement technology.